In recent years, research and development has progressed in organic electro-luminescent elements (organic EL elements), which make use of the electro-luminescence of organic material and are one type of organic light-emitting element. A typical organic EL element includes an anode, a cathode, and a light-emitting layer (an organic layer) between the anode and the cathode.
An organic EL display panel includes a two-dimensional array of pixels. Each pixel includes a plurality of sub-pixels. In an organic EL display panel, adjacent sub-pixels are partitioned from one another by banks that contain electrically-insulative material and define the shape of light-emitting layers in the sub-pixels.
Each sub-pixel (organic EL element) in an organic EL display device may include layers other than the electrodes and the light-emitting layer, as necessary. For example, a combination of a hole injection layer and a hole transport layer, or a hole injection/transport layer may be disposed between the anode and the light-emitting layer. For example, a combination of an electron injection layer and an electron transport layer, or an electron injection/transport layer may be disposed between the cathode and the light-emitting layer. In the following, the term “charge injection/transport layer” is used. A charge injection/transport layer may be any of: a hole injection layer; a hole transport layer; a hole injection/transport layer; an electron injection layer; an electron transport layer; and an electron injection/transport layer. Further, in the following, the term “functional layer” is used. A functional layer may be a light-emitting layer or any charge injection/transport layer. The term “functional layer” is used to refer to such layers because such layers each have a unique function. Specifically, a light-emitting layer has the function of emitting light, and a charge injection/transport layer has the function of injecting or transporting charge.
The manufacturing of an organic EL display panel includes a process of forming a functional layer above a substrate. Note that in the present disclosure, when a first layer is disposed “above” a second layer, the first layer may be disposed directly on the second layer so as to be in contact with the second layer, or may be disposed above the second layer without direct contact with the second layer. Typically, a functional layer is formed by using low molecular weight material and by performing vapor deposition. However, a functional layer may also be formed by performing a so-called “wet process”. Forming a functional layer through a wet process involves first applying ink (application liquid) to fill a recess between banks through inkjet printing, and then drying the ink so applied. The ink contains functional layer material dissolved in a solvent. A wet process facilitates the forming of functional layers, even those in organic EL display panels of large size.
The thickness of a function layer is one factor having influence on light-emission characteristics of an organic EL element. Due to this, when forming functional layers in an organic EL display panel through a wet process, it is preferable that recesses between banks are filled with a similar amount of ink, and that a functional layer formed with respect to each recess has uniform thickness and a high level of planarity.
Conventionally, ink for a functional layer includes a solvent having a high boiling point. To cause such a solvent to evaporate, after being applied to fill recesses above a substrate, the ink is dried by decompression-drying the substrate in a drying machine.